Hey - Congrats on the new job. Do what makes you feel most comfortable, perhaps forewarn your manager/colleagues that you have a stammer and even mention it on the phone if that makes you feel better. The best advice I can offer, and that is coming from someone in a customer service position who used to do as much as possible to try and avoid phone calls etc, is to put yourself outside of your comfort zone and be the person to make the calls/answer the calls. Once you’ve got the hang of it you’ll feel more confident which will in turn help your fluency. Good luck buddy!